Objective-c 字面量

Objective-c早就支持字面量，但是IOS到XCODE 4.5，IOS6，LLVM4.0才开始较好的支持字面量。

以下是简要对比：

未使用字面量	使用字面量
NSString *greeting = [NSString stringWithUTF8String:"Hello  World"];	NSString   *greeting = @"Hello World";
NSNumber *number;	NSNumber  *number;
number = [NSNumber numberWithChar:'X'];	number  = @'X';
number = [NSNumber numberWithInt:12345];	number  = @12345;
number = [NSNumber numberWithUnsignedLong:12345ul];	number  = @12345ul;
number = [NSNumber numberWithLongLong:12345ll];	number  = @12345ll;
number = [NSNumber numberWithFloat:123.45f];	number  = @123.45f;
number = [NSNumber numberWithDouble:123.45];	number  = @123.45;
number = [NSNumber numberWithBool:YES];	number  = @YES;
[NSNumber numberWithint:6 + x * 2012]	@(6+x*2012)
NSNumber *piOverSixteen = [NSNumber numberWithDouble: (M_PI /  16)];	NSNumber  *piOverSixteen = @( M_PI / 16 );
NSNumber *hexDigit = [NSNumber  numberWithChar:"0123456789ABCDEF"[i % 16]];NSNumber *hexDigit = @(  "0123456789ABCDEF"[i % 16] );	NSNumber  *hexDigit = @( "0123456789ABCDEF"[i % 16] );
NSNumber *usesScreenFonts = [NSNumber  numberWithBool:[NSLayoutManager usesScreenFonts]];	NSNumber  *usesScreenFonts = @( [NSLayoutManager usesScreenFonts] );
NSNumber *writingDirection = [NSNumber  numberWithInt:NSWritingDirectionLeftToRight];	NSNumber  *writingDirection = @( NSWritingDirectionLeftToRight );
NSNumber *path = [NSString stringWithUTF8String:  getenv("PATH")];	NSNumber  *path = @( getenv("PATH") );
array = [NSArray arrayWithObjects:a, b, c, nil];	array  = @[ a, b, c ];
[array objectAtIndex:1]	array[1]
dict = [NSDictionary dictionaryWithObjects:@[o1, o2, o3]   forKeys:@[k1, k2, k3]];	dict  = @{ k1 : o1, k2 : o2, k3 : o3 };
[NSDictionary dictionaryWithObjectsAndKeys: v1, k1, v2, k2,  nil];	@{k1:v1,  k2:v2}
[dictionary valueForKey:k]	dictionary[k]

注：

@YES/@NO和下标功能从IOS6才开始支持（XCODE4.5+LLVM4.0）。

其他字面量从XCODE4.4+LLVM4.0就已经支持。